'Eve' May Have Been The Mother Of Us All

A Study Published Today Supports The Theory That All Humans Share A Single Common Ancestor.

September 27, 1991|By Philadelphia Inquirer

PHILADELPHIA — She lived in the African savanna 200,000 years ago with a band of 25 to 50 other primitive humans. She used rudimentary stone tools, may have lived in a cave and competed with lions for scavenged food.

She is also - this single, particular woman - the common ancestor of every living human being on Earth today.

This controversial idea, dubbed the Eve hypothesis, has been debated by scientists since it was first proposed in 1987.

Today it gets its strongest support yet, in the form of new genetic evidence put forward by a team of scientists at Pennsylvania State University and two other colleges. In a paper in the journal Science, they trace humans back to a single African woman who, they say, lived between 166,000 and 249,000 years ago.

''Our findings confirm the original hypothesis,'' said Henry Harpending, a Penn State anthropologist and a co-author. ''In science you never believe one report until it has been confirmed by additional studies. This study confirms the original theory.''

But even as the paper was being published, other scientists said they were still skeptical.

Harpending and the study's other co-authors point out that their hypothetical Eve - unlike the biblical one - was in no sense the single ancestral mother of all people. There were other women having babies at the time the scientific Eve lived.

Eve, though, would be the last direct common ancestor of everyone alive today.

The scientists reached this conclusion by studying a form of genetic material known as mitochondrial DNA (mtDNA).

Mark Stoneking, a Penn State geneticist who co-authored the study, said that during the last four years there has been mounting evidence - both in the form of fossils and genetic analyses - to support the view that all of the 5.5 billion people on Earth are direct descendants of one African woman.

Stoneking said the Eve hypothesis supports the school of thought that bands of hunter-gatherers began migrating out of Africa about 200,000 years ago, spilling over into Europe and Asia. In the process, they replaced other primitive humans who already lived in those regions.

The new findings are expected to stir up debate once again over how humans evolved. And not all scientists believe the findings being reported today by researchers at Penn State, the University of Utah and the University of California at Berkeley.

''Their analysis is based upon a series of assumptions, most of which are unverifiable,'' said Erik Trinkaus, an anthropologist at the University of New Mexico. ''I'm not saying they are wrong about a common African origin, but they haven't yet proven they are right. I think their findings are built on a house of cards.''

Unlike conventional DNA, which is passed down to offspring by both parents, mtDNA is inherited solely through the mother.

The only way mtDNA can change from one generation to the next is through mutations. And since many scientists think the mutation rate proceeds at a steady, known rate, they can compare differences in the mtDNA of people living today and use it to reconstruct their evolutionary history.

In today's study, the researchers examined the mtDNA found in 189 people living in Africa, Europe, Asia, Australia, Papua New Guinea and North America. They closely examined variations in tiny sub-units of the mtDNA.

Then, using computers, they analyzed the differences and constructed a worldwide evolutionary tree. The trunk of this tree, by their analysis, seemed to be in sub-Saharan Africa.

And since the mutation rate for mtDNA is known, they also tried to calculate the time when the tree first began to branch - which would be when our oldest common ancestor lived.

Trinkaus said that while the Eve hypothesis continues to be hotly debated, most scientists agree on certain facts about early humans:

''An overwhelming number of anthropologists believe that human lineage originated 6 (million) to 10 million years ago in Africa. They evolved for a long time in Eastern and Southern Africa, and about 1 million years ago had spread throughout Africa and across Southern Asia,'' he said.

''Gradually, over the next 500,000 years they spread into Europe and Northern Asia. People didn't make it into the Americas until 20,000 to 30,000 years ago when they could walk across the Siberian land bridge to North America.''